Provide an environment flag to disable error backtrace capturing. (#812)

In performance-sensitive environments that throw many errors, the cost
of capturing a backtrace for each one may be prohibitive. This PR allows
disabling backtrace capture in environments where this is a problem.

Resolves #791.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/SourceAttribution/Backtrace.swift

@@ -353,19 +353,25 @@ extension Backtrace {
   /// This value is named oddly so that it shows up clearly in symbolicated
   /// backtraces.
   private static let __SWIFT_TESTING_IS_CAPTURING_A_BACKTRACE_FOR_A_THROWN_ERROR__: Void = {
-    _ = isFoundationCaptureEnabled
-
-    _oldWillThrowHandler.withLock { oldWillThrowHandler in
-      oldWillThrowHandler = swt_setWillThrowHandler { errorAddress in
-        let backtrace = Backtrace.current()
-        _willThrow(errorAddress, from: backtrace)
-      }
+#if SWT_TARGET_OS_APPLE && !SWT_NO_DYNAMIC_LINKING
+    if Environment.flag(named: "SWT_FOUNDATION_ERROR_BACKTRACING_ENABLED") != false {
+      _ = isFoundationCaptureEnabled
     }
-    if #available(_typedThrowsAPI, *) {
-      _oldWillThrowTypedHandler.withLock { oldWillThrowTypedHandler in
-        oldWillThrowTypedHandler = swt_setWillThrowTypedHandler { errorAddress, errorType, errorConformance in
+#endif
+
+    if Environment.flag(named: "SWT_SWIFT_ERROR_BACKTRACING_ENABLED") != false {
+      _oldWillThrowHandler.withLock { oldWillThrowHandler in
+        oldWillThrowHandler = swt_setWillThrowHandler { errorAddress in
           let backtrace = Backtrace.current()
-          _willThrowTyped(errorAddress, errorType, errorConformance, from: backtrace)
+          _willThrow(errorAddress, from: backtrace)
+        }
+      }
+      if #available(_typedThrowsAPI, *) {
+        _oldWillThrowTypedHandler.withLock { oldWillThrowTypedHandler in
+          oldWillThrowTypedHandler = swt_setWillThrowTypedHandler { errorAddress, errorType, errorConformance in
+            let backtrace = Backtrace.current()
+            _willThrowTyped(errorAddress, errorType, errorConformance, from: backtrace)
+          }
         }
       }
     }